M635E NTN Blueprint Demo

An always-on satellite (NTN) demonstration blueprint for the Particle M635e platform. This application demonstrates LTE-M ↔ NTN fallback behaviour for permanently powered devices, and doubles as a rich NTN diagnostic tool for evaluating satellite connectivity, antenna placement, and network transitions.

Note: This is intentionally an always-on demo. Power management, deep sleep, battery optimisation, and low-power modem operation are excluded from this release to reduce complexity and maximise visibility into system behaviour.

Features

• LTE-M operation with periodic publishing
• NTN (satellite) operation with location-assisted acquisition
• LTE → NTN fallback when LTE publishes have failed beyond a configured timeout or LTE connectivity is not available
• NTN → LTE recovery via periodic LTE retry
• eSIM profile switching between LTE and NTN profiles
• Single publish abstraction that routes to LTE or NTN automatically
• Verbose USBSerial diagnostics — signal quality, satellite acquisition, state transitions, publish results, and timing estimates
• Simulated LTE loss mode for testing fallback without leaving coverage
• Hardware watchdog protection against stuck states
• Prebuilt NTN-first binary for immediate experimentation

Requirements

Hardware

• Particle M635e (M-SoM with NTN support)
• NTN-capable antenna (see Particle docs for more info)
• Optional GNSS antenna
• USB connection for USBSerial debug output
• Permanent power supply (this is an always-on application)

Software

• Device OS 6.4.2 or later
• eSIM profiles for both LTE and NTN installed on the device (profiles may or may not be activated — the application handles both cases). These are installed when going through setup.particle.io
• Does not depend on the Location API

Quick Start

1. Connect the NTN antenna to the CELL connector on the M635e SOM.
2. Go to blueprints.particle.io/ and select "Use this blueprint" and then "Deploy to my device"
3. Go through the process at setup.particle.io to configure your device, install the eSIM profiles and finally, this application
4. Open a serial terminal to view USB Serial debug output ("particle serial monitor" from the CLI). As soon as programming is completed, it will begin to search for NTN satellites and connect to the Particle cloud. The default configuration is NTN-first / NTN-only publishing, so satellite behaviour is demonstrated immediately.
5. Make sure you have a clear view of the sky (outdoors) for the device to connect. Initial Satellite registration can take up to 10 minutes.

Customizing the Demo

1. Download this blueprint from github and open in Particle Workbench
2. Select M-SoM as the device type and then compile and upload the program by plugging in the device to your computer over USB
3. Modify the env.json parameters to update if you are publishing from cellular, satellite or both!

On every boot the application resets all timers and the state machine, enables the watchdog, and reinitialises the modem.

Antenna Guidance

NTN connectivity is highly sensitive to antenna selection and placement:

• Use an antenna rated for the NTN band with clear sky visibility.
• Place the antenna with an unobstructed view of the sky — away from metal enclosures, windows with coatings, and overhangs.
• Use the UART satellite acquisition and signal-quality output to compare placements empirically — this application is designed as a placement-evaluation tool.

Configuration

All runtime behaviour is defined in a single top-level env.json file. Workbench builds each key/value pair into the application binary as an environment variable, and the firmware reads them at boot (see src/app_config.cpp). Any variable that is missing or invalid falls back to the compiled default in src/app_config.cpp.

Key	Type	Default	Meaning
INITIAL_ONLINE_TIMEOUT_S	uint	0	If > 0, connect to the Particle cloud once at boot. Stays online for this many seconds, then disconnect and proceed to the normal startup flow. Useful to ensure the cloud can update device state and environment variables. Can be 0 when if setting PARTICLE_LOCATION_FIXED directly, or when using setup.particle.io.
FEATURE_LTE_ENABLED	bool	false	Allow LTE-M as a connectivity stack.
FEATURE_NTN_ENABLED	bool	true	Allow Satellite NTN as a connectivity stack. At least one of the two FEATURE_*_ENABLED flags must be true; LTE will be enabled if both are false.
START_ON_CELLULAR	bool	false	Which radio the device boots on. true = LTE-M; false = NTN (useful for NTN-first demos).
CONSTRAINED_PROTOCOL_ON_CELLULAR	bool	false	Testing aid. When true, publishes made while on the normal Device OS connection (LTE-M, or WiFi) are routed through the constrained protocol — the same CloudProtocol / secure-UDP datagram stack used over NTN, sent to the NTN ingress — instead of Particle.publish. Exercises the exact NTN wire protocol (framing, counters, tags, ACKs, queued downlinks) without waiting for a satellite attach. The Particle cloud session stays up (OTA/console keep working) and the NTN rate limit and payload cap still apply. No fallback: if the secure-UDP session can't initialize, publishes fail visibly in the publish stats.
LTE_PUBLISH_INTERVAL_S	uint	60	Seconds between publishes while on LTE-M.
NTN_PUBLISH_INTERVAL_S	uint	180	Seconds between publishes while on NTN. Cannot be set below 30.
VITALS_INTERVAL_S	uint	600	Seconds between periodic device-vitals publishes. Vitals are always published once on (re)connect regardless of this value; 0 disables the periodic refresh (on-connect only).
NTN_MAX_PAYLOAD_SIZE	uint	256	Max on-wire datagram size for outbound NTN publishes, including the 9-byte Secure UDP overhead when enabled (the protocol frame gets the remainder). Clamped to the transport maximum of 256.
CELLULAR_DISCONNECTED_TIMEOUT_S	uint	600	Seconds disconnected on LTE before switching to Satellite. There is no cellular "connected" timeout — if LTE is up, we stay.
SATELLITE_CONNECTED_TIMEOUT_S	uint	600	Seconds connected on Satellite before switching back to test Cellular again.
SATELLITE_DISCONNECTED_TIMEOUT_S	uint	600	Seconds disconnected on Satellite (including while still acquiring — SEARCH/LIMSRV before attach) before switching back to Cellular. NTN attach can take minutes, so don't set this too low or the device gives up before it ever connects.
FORCE_CELLULAR_TO_SATELLITE_SWITCH	bool	false	Bench testing only. When true, the LTE→NTN switch fires purely on FORCE_C2S_SWITCH_TIMEOUT_S after radio enable, ignoring LTE connection state.
FORCE_SATELLITE_TO_CELLULAR_SWITCH	bool	false	Bench testing only. When true, the NTN→LTE switch fires purely on FORCE_S2C_SWITCH_TIMEOUT_S after radio enable, ignoring NTN connection state.
FORCE_C2S_SWITCH_TIMEOUT_S	uint	300	Force-mode timeout for the LTE→NTN switch. Ignored unless FORCE_CELLULAR_TO_SATELLITE_SWITCH is true.
FORCE_S2C_SWITCH_TIMEOUT_S	uint	300	Force-mode timeout for the NTN→LTE switch. Ignored unless FORCE_SATELLITE_TO_CELLULAR_SWITCH is true.
USE_ONBOARD_GNSS_FOR_LOCATION	bool	false	Where the NTN location fix comes from. false = use the PARTICLE_LOCATION_FIXED coords below; never query the GNSS engine (no-antenna devices). true = use the onboard GNSS engine for up to ONBOARD_GNSS_FIX_TIMEOUT_S, then fall back to those coords.
ONBOARD_GNSS_FIX_TIMEOUT_S	uint	300	Maximum seconds to wait for a GNSS fix when USE_ONBOARD_GNSS_FOR_LOCATION is true before giving up and using the fixed coords. Unused when it is false.
PARTICLE_LOCATION_FIXED	string	"44.92653,-93.39767,283"	Fixed location as "<latitude>,<longitude>,<altitude>" in decimal degrees / meters. Used directly when GNSS is disabled, and as the fallback when GNSS is enabled. Warned about if missing/invalid while USE_ONBOARD_GNSS_FOR_LOCATION is false.

Example Configurations

NTN-only (default shipped config) — demonstrates satellite connectivity immediately:

• LTE disabled, NTN enabled, NTN-first publishing.

LTE-only — baseline cellular operation:

• LTE enabled, NTN disabled.

Hybrid (LTE primary, NTN fallback) — real-world deployment pattern:

• Both enabled. LTE is used until no successful publish occurs within the failure timeout (default 15 min), then the device switches eSIM profile and attaches to NTN. LTE is retried periodically; on restoration the device returns to LTE.

Use the fake LTE loss option to exercise the hybrid fallback lifecycle without leaving LTE coverage.

How It Works

Connectivity Lifecycle

If INITIAL_ONLINE_TIMEOUT_S > 0, the device runs InitialConnect → InitialOnline first before moving to Start.

                              ┌───────┐
                              │ Start │
                              └──┬────┘
                                 │ radioEnable(start radio)
                                 ▼
                       ┌──────────────────┐
                       │ AcquireLocation  │      (one-shot, at boot only)
                       └──────────┬───────┘
        radioEnabled() == CELLULAR│ (else: SATELLITE)
                  ┌───────────────┴───────────┐
                  ▼                           ▼
        ┌──────────────────┐        ┌──────────────────┐
        │ CellularConnect  │        │ SatelliteConnect │
        └────────┬─────────┘        └────────┬─────────┘
                 │ Particle.connected()      │ satellite.connected()
                 ▼                           ▼
        ┌──────────────────┐        ┌──────────────────┐
        │  CellularOnline  │        │ SatelliteOnline  │
        └────────┬─────────┘        └────────┬─────────┘
                 │ cellularShouldSwitch      │ satelliteShouldSwitch
                 │ ToSatellite()             │ ToCellular()
                 ▼                           ▼
        ┌──────────────────┐        ┌──────────────────┐
        │ SwitchToSatellite│        │ SwitchToCellular │
        └──────────────────┘        └──────────────────┘
        radioEnable OK              radioEnable OK
        → SatelliteConnect          → CellularConnect

   (any radioEnable / satellite.begin() failure → Fault → Start)

LTE failure definition: LTE is considered unavailable when no successful LTE publish occurs within the configured timeout window — even if registration, attach, or partial connectivity succeeds.

Publish Abstraction

The application exposes a single publish call that routes to the LTE or NTN stack based on the current operating mode:

publisher.publish("event_name", eventData);

This layer centralises rate limiting (1 message / 30 s on NTN), payload sizing (256-byte NTN limit), retries, logging, metrics, and publish accounting. Message types include vitals payloads, event publishes, and subscribes (all rate-limited on NTN).

Location Handling

NTN attach requires location. USE_ONBOARD_GNSS_FOR_LOCATION selects how it is obtained:

1. false — use the PARTICLE_LOCATION_FIXED coordinates ("lat,long,altitude") directly; the GNSS engine is never queried (no-antenna devices).
2. true — use the onboard GNSS engine for up to ONBOARD_GNSS_FIX_TIMEOUT_S, then fall back to the PARTICLE_LOCATION_FIXED coordinates if no fix is obtained.

Acquired location is cached and reused on each NTN attach attempt.

Watchdog & Recovery

The application watchdog protects against stuck connection state machines, blocked attaches, blocked location acquisition, profile-switching hangs, and publish loop stalls. Repeated attach failures escalate to a modem reset.

LED Behaviour

When the NTN radio is enabled, the system LED is overridden to clearly signal NTN acquisition status:

• Solid Green: NTN is searching
• Solid Cyan: NTN is successfully registered When on Cellular, the LED retains the normal particle behavior (ie blinking green for network registration, breathing cyan once registered and connected to the Particle cloud)

Debug Output Interpretation

The USBSerial output is designed as a continuous NTN diagnostic environment. Expect to see:

• Startup banner — full runtime configuration at boot
• Operating mode — current mode (NTN vs LTE) reported continuously
• Signal metrics — LTE RSSI/RSRP/SINR and NTN signal quality where available
• Satellite activity — acquisition progress and visible satellites
• State transitions — connection state machine changes, eSIM profile switches, modem transitions
• Publish results — attempts, successes, failure causes, and success/failure counters
• Timing estimates — retry windows and time remaining before fallback/recovery transitions
• Location status — fixed vs acquired, acquisition progress

The goal is that you can understand what the modem is doing even when an NTN connection is unsuccessful.

NTN Operational Expectations

• Satellite acquisition can take significantly longer than LTE attach; the timing-estimate log lines show where the device is in the process. First attach can take ~10 minutes.
• NTN payloads are limited to 256 bytes and 1 message per 30 seconds — the publish layer enforces both.
• NTN latency per packet is on the order of ~10 seconds per message.
• A valid location is required before NTN attach.
• Clear sky view is essential; use the diagnostic output to validate placement.

Skylo Conformance

This application is expected to serve as the demo app for Skylo conformance testing. The primary design goal is correct customer-facing behaviour; a conformance-specific branch may diverge as needed. The shipped configuration undergoes a Skylo compliance review.

Troubleshooting

Symptom	Things to check
No NTN attach	Sky visibility, antenna selection/placement, valid location available, NTN eSIM profile installed
Stuck during attach	Watchdog and modem-reset escalation will recover automatically; check state transition logs for the failure cause
Publishes failing on LTE	Expected to trigger NTN fallback after the timeout — check publish failure log lines for cause
Device never falls back to NTN	Verify NTN is enabled in config and the LTE failure timeout; use fake LTE loss mode to force the transition
No location	Set a fixed location in config, or ensure GPS has sky view

License / Support

Refer to Particle documentation and support channels for the M635e and NTN platform.